Digital communication system



April 9, 1968 R. F. J. FILIPOWSKY DIGITAL COMMUNICATION SYSTEM Filed Oct. 7, 1966 ll Sheets-Sheet 1 MASTER TIMER ,4 INPUT PRODUCT ENCODER WAVEFORM TRANSMITTER GENERATOR OUTPUT RECEIVER WAVEFORM DECODER 32 22 X FROM I0 TRANSMITTER ENCODER 23 I X smx I O ccsx 0 sm 2x 0 FREQUENCY 905 H w SYNTHESIZER E cos 3x 0 sm 4): cos 4x 6 I/VI/EIVTOR FROM TIMING DEVICE FIG. 2

RICHARD F. J. FILIPOWSKY By ffllwi A T TOR/VF Y A ril 9, 1968 R. F. J. FILIPOWSKY DIGITAL COMMUNICATION SYSTEM Filed Oct. 7, 1966 11 Sheets-Sheet FROM RECEIVER 4o COMPOSITE /60 41 EXTRACTOR X ,6: 16 71 S an .1 X s2 1 s 63 DECISION 33 T DEVICE DECODER r44 s K 19 smx cosx SIM J rmmc 3 FREQUENCY cos 3x SYNTHESIZER sm4x C05 4X 3 FROM RECEWER 40 COMPQSITE 61 EXTRACTOR n 77' 45 81 H. x S THRESHOLD 42 DEVICE I A x F. THRESHOLD 12 3 THRESHOLD DEVICE S THRESHOLD DEVICE rs smx 905x TIMING sm2x C052) FREQUENCY F sm 5x SYNTHESIZER cossx smax i H6. 4 cos4x April 1968 R. F. J. FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM Filed Oct. 7, 1966 v 11 Sheets-Sheet 5 GA PU 5 SWWOT.

COS w t SIN 2w t C05 Zw t SIN 5w t C05 SW01 a SIN 4w t cos 4 April 9, 1968 R. F. J. FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM Filed Oct. 7, 1966 r 11 Sheets-Sheet 4 V cos 2w t cos w t cos 2w t sm 4w t sm 2w t cos W 1 FIG. 6

April 9, 1968 R. F. J. FILOPOWSKY 3,377,525

DIGITAL COMMUNICATION SYSTEM 11 Sheets-Sheet 5 Filed 001.- 7. 1966 oazz C523 April 9, 1968 R. F. J. FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM 11 Sheets-Sheet Filed Oct.

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m w m L m G) If) P 1968 R. F. J. FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM Filed Oct. 7, 1966 11 Sheets-Sheet 1' |,1,2 WAVEFORM 1 WAVEFORM 2 WAVEFORM 3 WAVEFORM 4 5,4,17 WAVEFORM 5 WAVEFORM s 4,3,? WAVEFORM 7 4,8,29 WAVEFORM 8 FIG. 9

APPil 1968 R. F. J. FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM Filed Oct. 7, 1966 11 Sheets-Sheet E FIG. 10

April 1968 R. F. J FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM Filed Oct. 7. 1966 11 Sheets-$heet FIG. 11

April 1963 R. F. J. FILIPOWSKY 3,377,625

DIGITAL COMMUNICATION SYSTEM Filed Oct. '7, 1966 ll Sheets-Sheet iv f I WM P1 j- 0 r P5 f o FIG. 12

DIGITAL COMMUNICATION SYSTEM ll Sheets-5heet 1 1 Filed Oct.

f (t)=cosw tsin ZW t-smBW L f 2(t) FIG. 13

United States Patent 3,377,625 DIGITAL COMMUNICATION SYSTEM Richard F. J. Filipowsky, Huntsville, Ala., assignor to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Continuation-impart of application Ser. No. 465,880,

June 22, 1965. This application Oct. 7, 1966, Ser.

22 Claims. (Cl. 340-172.5)

ABSTRACT OF THE DISCLOSURE Digital communications systems employing finite sets of highly distinguishable transmission waveforms for transmitting information, each waveform being defined by the product of two or more harmonically related sinusoids, comprising: a transmitter including means for encoding each elemental message portion into one of the predetermined set of wideband waveforms wherein the waveforms are defined by the product of a plurality of harmonically related sinusoids over a given period of the fundamental frequency, a communication channel, and a receiver including optimum detection means for detecting which members of the set of wideband waveforms were transmitted.

This application is a continuation-in-part of application Ser. No. 465,880, filed June 22, 1965, now abandoned. The invention relates to digital communication systems employing finite sets of highly distinguishable transmission waveforms for transmitting information. More particularly, this invention relates to digital communication systems wherein the transmission waveforms are defined by the product of two or more harmonically related sinusoids.

The majority of the prior art digital communication systems employ analog techniques for the transmission of signals. In such communication systems, the rectangular digital waveforms, such as are used in digital computers, serve to modulate a carrier wave by means of well known amplitude, frequency, or phase modulation techniques. At the receiver a suitable analog demodulator operates on the modulated carrier wave to recover the original rectangular waveform as accurately as possible. Such communication systems are subject to the disadvantage that the digital waveforms which are finally reproduced at the output of the receiver are affected not only by the actual digital information transmitted, but also by the instantaneous phase difference between the carrier and the system clock, as well as on the detailed filter characteristics of all the filters in the transmission channel, and on random noise.

Communication systems using truly digital transmission techniques possess certain advantages over the prior art systems. In such communication systems, the rectangular digital waveforms serve to select members of a predetermined finite set of highly distinguishable transmission Waveforms. The pre-knowledge of the characteristics of each member of the set of transmission waveforms can be used to design receiving apparatus including a set of optimum detection devices, each of which is responsive to only one member of the set of transmission waveforms. Hence, in spite of the fact that the predetermined transmission waveforms are subject to the inevitable distortion and random noise factors, each optimum detection device is capable of making the simple decision as to whether it is more probable than not that its associated transmission waveform was received during a particular transmission interval.

From the foregoing it is apparent that digital communication systems using predetermined sets of transmission waveforms in combination with optimum detection devices are relatively insensitive to noise and channel distortion. This characteristic is most pronounced when the members of the set of transmission waveforms are highly distinguishable from each other. Waveforms having a distinguishability of the highest possible order are known as orthogonal waveforms. Therefore, digital communication systems using orthogonal waveforms in combination with optimum detection devices would be expected to deliver superior performance in the presence of large amounts of random noise and channel distortion.

In digital communication systems using large sets of predetermined transmission waveforms each waveform may carry several bits of information. Such communication systems are called n-ary" systems or high order alphabet systems. If the set of transmission waveforms is carefully chosen, such n-ary" communication systems can provide extremely efiicient utilization of the avail able channel bandwidth.

A chief obstacle to the full exploitation of the benefits of truly digital transmission techniques has been the difficulty and expense of generating large sets of highly distinguishable transmission waveforms. It has proven especially difficult to generate large sets of orthogonal waveforms. Moreover, prior attempts to generate such waveforms have usually resulted in waveforms having sharp discontinuities. Waveforms having sharp discontinuities are undesirable because they require transmission channels of very large bandwidth in order to avoid serious distortions.

It is, therefore, an object of this invention to provide improved means for generating very large sets of highly distinguishable waveforms.

Another object of this invention is to provide means for generating large sets of orthogonal waveforms.

It is also an object of this invention to provide means for generating sets of orthogonal waveforms which have few sharp discontinuities.

It is still another object of this invention to provide sets of band-limited waveforms which can be directly generated for use in pass band carrier channels and which do not require a separate oscillator and modulator for being converted from a base-band to a carrier band.

I have discovered that the above objects are satisfied by certain waveforms defined by the product of two or more harmonically related sinusoidal functions taken over an interval equal to one-half the period of the fundamental frequency. Large sets of such waveforms, hereinafter designated product waveforms, may be generated from a relatively small number of harmonically related sinusoids. Furthermore, each full set of product waveforms will contain several subsets whose members will be orthogonal to each other. In addition, product waveforms possess few discontinuities, and are bandlimited so as to permit eflicient utilization of the available bandwidth of the communication channel.

It is, therefore, an object of this invention to provide means for generating product waveforms.

It is also an object of this invention to provide a digital transmission system using product waveforms.

In accordance with the above objects, I provide a digital transmission system using product waveform generators in combination with optimum detection devices. In the transmitter I provide means for generating a plurality of harmoncially related sinusoids, means for multiplying selected groups of said sinusoids, and means responsive to the input data for gating the resulting product waveforms into the transmission channel. At the receiver, I provide means for generating a number of harmonically related sinusoids, and means for multiplying selected groups of said sinusoids in order to produce product waveforms corresponding to the product waveforms generated at the transmitter. I further provide cross-correlation means for cross-correlating the received signals with each of these product waveforms. Such cross-correlation means might comprise means for multiplying the received signals wlth each of the product waveforms generated at the receiver, and means for integrating each of the resulting products. A decision device responsive to the cross-correlation means is provided for determining which of the product waveforms were transmitted.

An advantage of my invention is the precision with which all the waveforms can be produced. Since all components of any waveform are harmonically derived from a fundamental frequency, it is easily possible to drive all generators and the synchronization devices from the same highly precise master timer.

Another advantage of my invention flows from the multiplicative formation of the waveforms. Because of this feature, the orthogonal character of an orthogonal subset of product waveforms is retained even if the individual components of the product waveforms contain small amplitude errors. Such amplitude errors will appear only as amplitude variations of the product waveform; not as waveform distortions. This is distinguished from the case of waveforms generated by the summation of components wherein an amplitude variation in the component will appear as a distortion of the final summation waveform.

A further advantage of my invention is the fact that each set of product waveforms contains at least one subset with members which restrict their energy to a smaller bandwidth than other subsets. This permits adaptive systems with variable bandwidth by the selection of subsets with the maximum number of waveforms filling the instantaneously available bandwidth; but not exceeding it.

Still another advantage of my invention is in providing a digital communication system using a set of highly distinguishable waveforms which may be readily expanded to take advantage of low noise environments.

Another advantage of my invention flows from the fact that a composite signal formed by superposing my product waveforms will have a rather uniform or noiselike spectral density over the larger part of its bandwidth. Communications theory has established that an efiicient transmission system should use signals having such a noiselike character.

Other objects and advantages of my invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode which has been contemplated of applying that principle.

FIG. 1 shows a block diagram of a digital communications system using product waveforms.

FIG. 2 shows a more detailed block diagram of a twofactor product waveform generator.

FIG. 3 shows a detailed block diagram of a two-factor product waveform detector adapted for use in a communication system using single digit operation.

FIG. 4 shows a detailed block diagram of a two-factor pro-duct waveform detector adapted for use in a communication system multi-digit operation.

FIG. 5 shows the phase relationship between the harmonically related sinusoids and the gating pulses.

FIG. 6 shows a sample orthogonal subset of the whole set of two-factor product waveforms which may be generated by the product waveform generator shown in FIG. 2.

FIG. 7 shows a detailed block diagram of the transmitting station of a digital communication system using multiple-factor product waveforms.

FIG. 8 shows a detailed block diagram of the receiving station of a digital communication system using multiplefactor product waveforms.

FIG. 9 shows time functions of typical multiplefactor product waveforms.

FIG. 10 shows time functions of an orthogonal subset of multiple-factor product waveforms.

FIG. 11 shows the Fourier spectra of the set of orthogonal waveforms shown in FIG. 10.

FIG. 12 shows the power spectra of the set of orthogonal waveforms shown in FIG. 10.

FIG. 13 shows two examples of orthogonal pass band waveforms and their spectra.

Before describing the detailed structure and operation of a specific communication system using my invention,

it is desirable to present a general discussion of the features and properties of product waveforms.

General discussion Expressed mathematically, the product waveforms utillzed in my communication system are defined by:

for

with

o f0fp.k.q(

where K is a positive integer indicating the highest harmonic of the fundamental frequency, f participating in a particular product waveform. K designates the class of the waveform. s s s s may assume the value 0 or 1; r r r, r may assume the value 0 or 1 (notice that the simultaneous selection of any s=r=0 leads necessarily to the trivial case: p is the number of non-trivial sine and cosine factors participating a particular product waveform, p designates the order of the waveform; q is a positive integer indicating the rank of a product waveform within a set of waveforms with equal p and k indices when all waveforms of such a set are placed in their natural" ranking. The natural rank of a product waveform is defined as its ranking list of product waveforms which starts with all sine functions and with the lowest possible grouping of indices and ends with all cosine functions and the highest possible grouping of indices. All terms are permuted from sine to cosine, starting with the term on the left hand end of the equation.

fits an example of the rank of waveforms, I list some typical waveforms of the class 8, order 4 in their natural sequence (with many omissions between the few examples).

6 Total number of waveforms of class 8 and order 4 is 35 X 16:560.

Equation 1 is sufficiently general to cover all possible un, i zx'sln sx'sm product waveforms. Naturally, Equation 1 contains, as f cos Imin ZXISIII ISM 8X 2 5 special cases, all trivial cases, such as j =l, which re- -,a,2 sults from the selection K=1; s O; r I. The simple I a 3 sin XICOS 2x sIn :IIXISIII RX 4 inelfunctiolns, 1 6 52; tlzlflor, resultsffrom thef selection I SK: r an e cosine unction, :cos I I w t, results from the selection of K l; s :1- r :1. 55, 8x III These two functions are the only functions in class oiie. f n Im n X COS 9 EX 8 The "level of a set of product waveforms is determined IBIS I I I I by the highest class of any waveform in the set but I I I I the set may contain waveforms of lower class. It can be 5 I cos X'cos Ices 9min Bx 16 seen from Table i that level 1 has a total of 3 waveforms, i I5 and level 2 a total of 9 waveforms. These 9 consist of the f a Sin xwin ZXIsin was 8K 3 waveforms of level 1 plus the two waveforms of first NIB? I I i order and second class plus the four product waveforms I I I l l of second order and second class. This can be continued, fII,|B I5 CD5 X105 zx-cos was ex recognizing that level 3 contains 27 waveforms and level 20 four 81 waveforms, each level automatically including all f I I sin IVSIII ZIPSIII ,III.5III 8X waveforms of all lower levels. Hence, it can be seen that the maximum number of product waveforms increases f I SI X s 2X I I I rapidly with an increase in level.

III a. In lll't co s 4x ctis ax 16 It is easy to see the general rule determining the maxi- I I I I I mum number of waveforms for any given level It. Each f I CI'IS pee; ZXICO'S a 8X term in Equation 1 may assume one of three states; sin 32 (kw l), cos (kw f), or 1. The maximum number of factors f, l I i MUSIIII 8x is equal to the highest value of k. The total number n of I I I I I 16 all possible and different multiple products of the k factors i i I I I is therefore the number of k-permutations of three things f I cos X'ces 3X-eus 4X'cos 8X l s IBIIII (states) with uniestiicted repetition (Ref. 12):

f sin 2X'sin SXsin IX-sin 8X 4 a, us I I I I (2) N=3 I l 15 i {I I H A .2 8x 35 Notice that the total number of waveforms for level 8 is therefore 3 6561, including the trivial case f :1; 5MB. I X' X' 8X 31 blocks of I6 including all the 81 waveforms of level 4 which are listed I I I I I wavefcms each in Table 1 and also including the complete set of 560 I I n CO5 was firms l 8x J 40 waveforms of class 8 and order 4 which has been used t above to demonstrate the ranking of waveforms.

TABLE I Number of Factors (Ultltlli cos X-eos 2X K23... l8 2? sin 3X sinX-sin 3X sin X-sin 2X sin 3X cus 3X (as X-sin 3X cos X-sin ZX-sin 3X sin X-cos 3X sin X-cns L'X-sin 3X cos X-cos 3X cos X-uos .ZX-sin 3X sin X-siii ZX-eos 3X cos \I .in 3X ens l t it X-cos 3X mus 3X sin i (us -X-uns 3X =4 .l 54 81 sin 4X sin X sin 4X sin X ZX-Sll14X cos 4X was k-sin 4X cos X S1Yl ZX-sin 4X sin X-ens 4X cos X-cos 4X sin QX-sit 4X Cos 2X-siii 4X (30s 4X ens X-ros 4X sin 3X-sin 4s sin 4X ens iikros 4X sin XGOS 2X sin 4X 00s Xros 2X-si'n 4X sin X-sin SX'sin 4X X sin 3X-Sill 4K in 4X cos X-cos liA-sin 4X sin X-sin BX-cos 4X ms is i EX-cos 4X sin X-cos 3X-cos 4X cos X-cos BX-cos 4X cos A in i.

cos X-c M ms X in in 3X 4X in 3X 4X TABLE OF 4TH LEVEL In addition to their large numbers, product waveforms possess a variety of advantageous properties. For example, FIG. 9 shows the large variety of waveforms which can be generated simply by multiplying sine or cosine functions of harmonically related frequencies. Waveform l is the above-mentioned simple cosine function, defined, as all the waveforms, from on a normalized w t axis. Notice that the definition of the time interval is rather arbitrary. For example, one could define the waveforms from Tr to +1r and use I as the lowest frequency and this would be identical with the definition above and using Zf as the lowest frequency, i.e., selecting 3 r l for all waveforms. The definition from on an m i axis is the most convenient one and it automatically includes all waveforms which could be defined for limits at integer multiples of Waveform 2 in FIG. 9 is an example of the first order waveforms which have been used many times in the prior art. They are truncated sine or cosine functions. In this case it is a waveform of exactly two full periods of a sine function. It results as the special case of a fourth class waveform, selecting in Equation 1 the following coefficients: s =s :s =0; r =r =r =l; s =l; r 0.

Waveforms 3 in FIG. 9 is an example of a second order waveform. This particular waveform results from the choice of s t); r l; s zl; r =l; s l; r :0. Table 1 shows that it has the rank 6 in the set of third class, second order waveforms. Therefore, it has the function symbol f (order 2, class 3, rank 6). This particular waveform is at the same time an example of an interesting set of waveforms which can only result when all sine terms have odd frequency multipliers and all cosine terms have even multipliers, i.e., if all r :0; r l. This particular subset of waveforms of any rank or order has the characteristic to have its peak values exactly at the end of the time interval where the waveform is being truncated. Although one would normally avoid such waveforms in communications systems because of their wide requency spectrum, good use can be made of members of this particular set as will be demonstrated hereinafter.

Waveform 4 in FIG. 9 is of a more general character than the previous ones. In this case, we have a waveform of third order and fourth class. It is a member of an orthogonal set of waveforms with particularly smooth characteristics. Such waveforms are discussed below in greater detail.

Waveform 5 in FIG. 9 is a typical example of a general waveform of third order. Table 1 indicates that there are 32 waveforms of third order, out of which 8 belong to class 3 and 24 to class 4. This particular waveform is a member of the class 4 set and has the rank 17. This example shows that the complexity of the waveforms increases with increasing order. But it should be noted that the WT product need not necessarily increase with increasing order. For example, there will always be members in the total set of waveforms of a given order which have a larger WT product than some members of the total set of waveforms of the next higher order. This is particularly true if the class of the waveform of lower order is high and that of the waveform of higher order is low. For example, sin 7X sin 8X has a larger WT product than sin X sin 2X sin 3X. This fact indicates that the extension of the sets of communications waveforms towards higher order multiple factor product Waveforms results in a true improvement in communications efficiency.

Waveform 6 in FIG. 9 is a member of the fourth order, fourth class set of 16 waveforms. In this set, as in all sets where class and order are equal, each waveform is a product of all possible harmonic factors, i.e., four factors in this case. Comparing waveform 6 with waveform 5 shows that the increase in order results in a further increase in complexity of the waveform as indicated by the larger number of zero crossings for waveform 6. Notice that both waveform 5 and waveform 6 belong to the same class.

Waveforms 7 and 8 of FIG. 9 are members of the particularly important set of orthogonal waveforms of fourth order and eighth class. Such sets of waveforms contain the maximum number of members for a given WT product, as explained below. It is also interesting to note that the larger number of factors used in the definition of these waveforms enables the simultaneous approach to zero value of the waveform and some of its derivatives. This fact permits the extremely smooth behavior of both these waveforms in the vicinity of 1:0. It also permits such waveforms to go through a zero value without changing polarity, as seen in waveform 7 around the 45 point.

Two-faotor product waveforms In order to more clearly illustrate the principles of my invention, it is convenient to refer initially to a communication system using product waveforms of only twofactors, i.e., product waveforms of the second order. Rewriting Equation 1 so as to express two-factor product waveforms, one obtains:

where fmkrqu) :0

T S s This determines that the fundamental frequency, 1 of the harmonically related sinusoids is given by In a practical communication system using second order product waveforms, the largest harmonic k of the fundamental frequency depends on the bandwidth (W) available. The product Waveform having the index m=k and n:kl will have a bandwidth (W) approximately equal to 2M Thus the time bandwidth (WT) product W W T is The subset of orthogonal waveforms In general, for any set of two-factor product waveforms having a time-bandwidth product WTgk there are several subsets having k orthogonal waveforms each. Usually there are several Ways of choosing these subsets of k orthogonal waveforms. For example, when k=4 the four waveforms with m=4; ":2 form the orthogonal subset shown in FIG. 6. It can be easily seen that all four waveforms will be orthogonal to each other, when the two symmetrical ones are mutually orthogonal and also the two skew symmetrical ones. Both statements can be easily verified if all these waveforms are foldable," and each half of them is again either symmetrical or skew-cylindrical.

In all, the set of 32 two-factor product waveforms for the case M54 contains 8 subsets of 4 orthogonal waveforms each. They are given below with numbers (l32) arbitrarily assigned to the waveforms and letters (A-H) assigned to the sets.

#11 sinw tcos 2w 1 0 l2 cos w t cos 2 #9 Sin wot Sill Zldot 10 cos w t sin 2w t Set D:

#13 sin w t sin 3m #14 cos w t sin 3w Set F:

#21 sin 0: 1 Slll 40.1 #22 cos w t sin 4 Set G:

# sin 2w sin 4w t #26 cos 2o 1 sin 40: 1

Set H:

#29 sin 3%! sin 4w f #31 sin Sw t cos 4 cos 3:0 1 sin 4m #32 cos 3w cos 4m It can be shown that the waveforms within each of the above subsets are orthogonal to each other over the range #15 sin w t cos Bo t #16 cos w cos 3w #19 sin 2 w t cos 301 2 #20 cos 2w t cos 301 1 #23 sin w r cos 2 #24 cos w t cos 4w #27 sin 210 f cos 4 #28 cos 2w cos 4w t but it is noted that the waveforms in one set are not necessarily orthogonal to the waveforms of any other set.

Taking advantage of the above grouping, it would seem that an efficient way of utilizing all 32 two-factor product waveforms would be to use all four product waveforms in each orthogonal subset to transmit information in the multi-digital mode. In the multi-digital mode all four waveforms are superposed during a single transmission interval T. Four bits of information are transmitted in this manner, each bit determining the polarity of one of the four waveforms. Additional information may be transmitted during each transmission interval by the selection of one of the eight orthogonal subsets. Such a selection carries ]og 8:3 bits of information. Hence, a

10 total of 7 bits of information can be transmitted in a signal cell having bandwidth-time product WT=4. This transmission rate is very close to the ideal Nyquist rate of 8 bits (ZWT) which heretofore was only theoretically achievable with sin x/x waveforms.

The above method of utilizing all 32 product waveforms may be further elucidated by way of the following example. Assume the following code is fixed for the selection of sets:

and assume further that the fourth to seventh bit of any seven bit word determines the polarity of the first to fourth waveform of the selected set. If in this system the seven bit word: has to be transmitted, the set G will be selected and the composite waveform will follow the equation:

This composite waveform will be correlated on the receiving side with all 32 product waveforms and two decisions will be made:

(a) A decision as to which set of waveforms has been used.

(b) A decision as to the polarity of the individual waveforms within this set.

The prior knowledge which the receiver can use is (a) the fact that for any message only product waveforms will be used, which belong to the same orthogonal set and (b) the fact that all of them will be used in every message.

The method described above for the special case of a set with 32 two-factor product waveforms (h1g4) can be applied in modified form to still larger sets.

T he subset of small band waveforms taken from a complete set of twoqactor product waveforms If m is large and n is very small in Equation 3, this results in waveforms with a very narrow frequency spectrum. Indeed these are the conventional double-sidebandsuppressed carrier signals and their spectrum is essentially limited to a bandwidth of It will be particularly useful to use such a subset over any channel with delay distortions. These are usually rather small over a fraction of the total band, but sizeable over the whole band. Small band waveforms will suffer much less distortion than wide band signals. Any absolute delay difference between the various sub-bands can easily be compensated for in the receiver by adjusting the de cision instant separately from sub-band to sub-band (waveform to waveform). Automatic adjustment of the resampling instant at the output of the matched filters in the receiver can therefore be used in place of any more complex automatic phase equalization scheme.

T he subset 0 1 wide band waveforms taken from a oomplete set of two-factor product waveforms Whenever I: is close to m the frequency spectrum of the waveform will be spread over a frequency band of nearly 2mi If m is also close to k, the wide band waveform will have components up to the extreme ends of the transmission bands. If both, In and n are very large, the spectrum will gradually turn into a line spectrum with one line at the lower end and one line at the upper end of the band.

Such wide band waveforms are desirable in many situations and they are already well known for special applications of signals with very large WT. Here, however, we are primarily concerned with WT from 2 to 20 and with fairly noise free channels, where large sets of non-orthogonal waveforms are desirable.

The subset of low pass waveforms, generally with DC components taken from a complete set of two-factor waveforms The smallef m, the larger may be the DC component which a waveform may possess. The extreme case of n:0, m:l is a product waveform with unilateral amplitude only. But even in the case "=3, "1:4, there is a a sizeable DC component. Such Waveforms may be of great value in channels with direct frequency modulation (video channels) where waveforms with their largest energy in the lower end of the band may be transmitted with a higher modulation index than waveforms with high frequency components. Further, an adaptive system may automatically select such sub-sets of waveforms whenever the channel might require a stronger noise reduction. The peak deviation of the modulator could also be adjusted accordingly.

The subset of transient waveforms taken from a complete set of two-factor product waveforms Examination of the 32 two-factor product waveforms set forth above for the case mg4 reveals that, for each choice of a special m and a special it there is one out of four waveforms which starts and ends with its peak value, thus causing a transient at the beginning and the end. While this behavior is normally undesirable, there are applications where such transient waveforms may be desirable for example for synchronization or for special character or word markers.

It is also possible to use a complete set of all waveforms, including the transient waveforms and to let a small gap between any two digits for the build-up and decay of the transient. If n and m of a transient waveform is small, but It is very large (W the bandwidth of the channel is relatively wide), the distortions of transient waveforms will be small enough to put them to good use.

Subsets 0f specified "distance" between the waveforms If orthogonal waveforms and non-orthogonal waveforms are used in the same system it is advisable to speak of the distance between these signals, making reference to an n-dimensional signal space with the orthogonal signals being the dimensions of this space (C. E. Shannon; Communication in the Presence of Noise; Proceedings of the IRE; vol. 37; January 1949; pp. l0-2l). Without going deeper into signal theory it may be stressed that the product waveforms offer the possibility to group all available waveforms into subsets of different distance.

The largest distance have signals from the subset of orthogonal waveforms, slightly smaller distance have waveforms which may be termed quasi-orthogonal, which have a very small cross-correlation coefiicient. In this manner, subsets of less and less orthogonality can be specified. The exact composition of these subsets has to be computed once M is chosen for any particular data transmission system.

In an adaptive data system is should be possible to add on more and more subsets with smaller and smaller distances between the waveforms whenever the channel transmission characteristics are good, and to drop back to sets with larger distances between the waveforms when the channel turns had again. An adjustment of all decision thresholds at the outputs of all matched filters at the receiving side must accompany this change in subsets.

Structure 0) system using two-factor product waveforms A block diagram of a digital communication system employing product waveforms is shown in FIG. 1. The original data is presented to the encoder 1 in either digital or analog form. The encoder 1 serves to encode each elemental portion of the input data into a pulse or group of pulses on the lines connecting the encoder 1 to the product waveform generator 2. These pulses determine which waveform or group of waveforms will be released to the transmitter 4 in any given interval and at which level and polarity for each waveform. The latter decision determines the value of the factor A in Equation 3. In a binary use of the waveforms the values of A will merely be +1 and l. In single digital operation, or in multiple digital operation, where the set of waveforms is perfectly orthogonal, it is also possible to use the waveforms in a non-binary multi-level mode. The number of levels will naturally be determined by the signal to noise ratio at the receiver.

It will be readily apparent that an encoder having the abovedescribed functional characteristics might easily be constructed by an engineer having ordinary skill in the art.

It is noted that a unitary mastertimer 3 controls both the encoder 1 and the product waveform generator 2. This coaction insures that the pulse outputs from the encoder 1 will be properly synchronized to the product waveforms so as to eliminate, insofar as possible, any discontinuities in the train of product waveforms appearing at the output of the product waveform generator, and to maintain the identifiability of the individual product waveforms at the product waveform detector 6. The specific synchronization requirements will be described in greater detail hereinafter.

The train of pro-duct waveforms appearing at the output of the product waveform generator 2 may be introduced directly into the transmission channel if desired. Alternatively, the train of product waveforms may be used to modulate a high frequency carrier. For example, if the particular transmission channel comprises a telephone line,

audio frequency product waveforms might be used directly without further modulation. On the other hand, if a radio frequency transmission channel is employed, the train of product waveforms would be used to modulate a radio frequency carrier.

On the receiving side of the system, the product waveform detector 6 uses matched filters or cross-correlators as signal extractors. Such signal extractors secure the well known noise improvement proportional to the time-bandwidth (WT) product of the signal. The product Waveform detector 6 additionally includes a decision device responsive to the signal extractors. This decision device must be programmed according to the mode of operation. In single digital operation it must select the highest output of any one of the extractors at the end of a digital interval unless a different decision rule is preferred. In multiple digital operation it must make individual decisions at the outputs of those extractors the waveforms of which are known to participate in the selected mode of operation.

All elements in this system are well known and have been extensively described in the prior art. The waveform generators can consist of oscillators, gates and multiplier circuits, which synthesize any of the selected waveforms according to Equation 3. This technique is well known from analog computers and signal simulators and is particularly easily applicable in this system, as all values of n and 111 must be integers, securing that all factors of all waveforms are harmonically related. In this case all oscillators will be phase-locked to the master timer and all must have a sine and a cosine output. There will be one multiplier required for each product waveform used in the system.

Another attractive method to generate such waveforms :as been described by G. McAuliffe: Impulsing of Linear Networks in Integrated Data Systems, IRE Transactions on Communication Systems, vol. CS7, No. 3, September 1959, pp. 189-194. It makes use of the fact that the impulse response of matched filters is the time inverse of the waveform to which they are matched. As

all of the waveforms of Equation 3 are either symmetrical or skew-symmetrical, one can use identically the same filters for excitation of waveforms at the transmitter as will be used as matched filters for the extraction of the waveforms at the receiving side. In a semiduplex operation (push to talk") it may be profitable to use physically the same filters for both purposes.

The matched filters themselves have been described in prior art and they may be either linear filters with active elements as described by McAuliffe, or use Fourier orthogonal filter techniques as described by Blasbalg: On the Application of Fourier Orthogonal Filters. Conference Proceedings. East Coast Conference on Aeronautical and Navigational Electronics, October 1958, pp. 165-172. The latter method is particularly attractive as the characteristics of any matched filter can easily be changed by altering the resistance matrices.

A detailed block diagram of the preferred form of the product waveform generator is shown in FIG. 2. It is noted that the preferred manner of generating product H waveforms is by means of a frequency synthesizer 10 comprising a plurality of phase-locked harmonic oscillators in combination with a plurality of product modulators 31-34. However. it will be understood that product waveforms may be generated by other means such as, for example, matched filters as mentioned above.

Although only four inputs 21-24 are shown in the preferred form of the product waveform generator, it will be understood that the practice of the subject invention is not limited to any specific number of inputs. For example, for the case m4 as shown in FIG. 2, 32 separate product waveforms may be generated. Hence 32 separate inputs might be profitably utilized. FIG. 2 shows only four inputs 21-24 in order that the principles of the invention may be illustrated in the simplest possible manner.

It is noted that the product waveform generator shown in FIG. 2 is suitable for use in both the single digital and multi-digital modes.

Referring now to the details of the product waveform generator shown in FIG. 2, a frequency synthesizcr 10 is provided for the purpose of generating a plurality of harmonically related sinusoids. More particularly. the fre quency synthesizer 10 generates both sine and cosine functions of a fundamental frequency and of each harmonic of that fundamental frequency up to and including the fourth harmonic. It should be noted, however, that the practice of the instant invention is not limited to the use of the eight harmonically related sinusoids shown in FIG. 2. Some communication systems will require larger numbers of harmonically related sinusoids, while in others, a smaller number would surface.

The details of construction of a frequency synthesizer of the type described would be readily apparent to one skilled in the art. For instance, a set of phased locked oscillators could be used as described in Chapter 2 of Radio Transmitters, by Lawrence Gray and Richard Graham (McGraw-I-lill, New York, 1961). Another pertinent reference in the frequency synthesis art is "A Survey of Frequency Synthesis Techniques," Milton Baltas, Army Electronics Research and Development Lab, Fort Monmouth, N.J., September 1962 (USAERDL Technical Report #2271).

A plurality of product modulators 31-3-1 are provided for the purpose of combining the harmonically related sinusoids so as to produce the desired product waveforms. These product modulators 31-34 may be of a conventional type known to those skilled in the art. In analog computers it is common practice to use time division multipliers for highest accuracy. (See for example: E. Kettel and W. Schneider: An Accurate Analog Multiplier and Divider; IRE Transactions on Electronic Computers, vol. EC 10, June 1961, pp. 269-272, containing references to older literature.) Such devices will be useful as product modulators in multi-digital applications of my invention, where highest accuracy of the product waveforms will be required to avoid intersymbol crosstalk. For less critical applications balanced modulators will be useful. (See for example: W. P. Birkemeier and G. R. Cooper: The Balanced Modulator As A Correlator for Random Signals; IRE Transactions Circuit Theory, vol. CT 9, December 1962, pp. 417-419.) Such modulators act as multipliers and produce suppressed carrier amplitude modulation. They have recently been designed with tunnel diodes as non-linear elements. (See: B. Rabinovici, T. Klapper and S. Kallus: Suppressed Carrier Modulations with Tunnel Diodes; Communications and Electronics, vol. 81, July 1962, No. 61, pp. 205-209.) For low frequency applications rectifier modulators are preferable. (See D. P. Howson: Rectifier Modulators, Analysis by Successive Approximations, Electronic Technology, vol. 37, April 1960, pp. 158-162. See also: D. P. Howson and D. G. Tucker: Rectifier Modulators with Frequency- Selective Terminations; Proc. Instn. El. Engrs, Part B, vol. 107, May 1960, No. 33, pp. 261-272.) Each product modulator 31-34 is associated with one of the inputs 21-24 from the encoder 1 shown in FIG. 1. Pulses appearing on the inputs 21-24 from the encoder serve to gate out product waveforms from the associated product modulators 31-34.

In the preferred form, the individual product modulators could be connected to the outputs of the frequency synthesizer by means of a patch board. A patch board would provide the communication system with a measure of flexibility in that it would readily allow different sets of product waveforms to be used. An inspection of FIG. 2 shows that only four of the eight harmonically related sinusoids are actually used to form the four product waveforms. 1n fact, the eight harmonically related sinusoids are capable of producing a set of 32 unique waveforms each of which is defined by the product of two harmonically related sinusoids as set forth above in the general description of the properties of product waveforms.

The 32 possible product waveforms which may be formed from the eight harmonically related sinusoids can be employed in a number of different ways. For example, in low channel noise conditions, a product waveform generator having 32 inputs from the encoder 1 may be used. In this way all 32 product waveforms can be utilized in a system transmitting in each time interval one waveform out of the set of 32 waveforms. This mode of operation is called the single digital mode. The use of all 32 waveforms in single digital mode is possible because, in low noise conditions, one can rely on slight differences in the correlation factors in order to distinguish the waveforms at the receiver.

On the other hand, when the channel noise level is high, an orthogonal subset of the 32 product waveforms should be chosen. The extremely high distinguishability of the waveforms in an orthogonal subset provides for error free identification of the transmitted waveform at the receiving end in the presence of high noise.

When medium noise levels are encountered, a set of quasi-orthogonal waveforms could be selected from the set of 32. The distinguishability of the quasi-orthogonal waveforms, and, hence, the size of the subset of such quasi-orthogonal waveforms, would be determined by the noise level in the particular channel. More particularly. the correlation of each quasi-orthogonal waveform with the other members of the set of quasi-orthogonal waveforms should be sufficiently low that the expected channel noise could not cause a positive correlation to be indicated when none, in fact, existed.

Still another way of employing the available product waveforms would be to utilize the multi-digital mode of operation. This could be accomplished by wiring the patch board to produce one of the eight subsets of four orthogonal waveforms. Superposition could be achieved by allowing a combination of gating pulses to appear on the input lines 2144 from the encoder during each time interval rather than the one gating pulse described above. Using the principle of superposition with four orthogonal waveforms, four bits of information could be carried by each composite product waveform. This represents an increase by a factor of two over the information carrying capacity of a communication system using a subset of four product waveforms in the single-digital mode. Naturally, this increase in information carrying capacity is counterbalanced by a requirement for lower noise conditions in order to provide error free performance when the multi-digital mode is employed.

It should be noted at this point that careful synchronization of the product waveform generator is required in order to achieve optimum performance of the communication system. More particularly, two kinds of synchronization are required. First, each of the eight harmonically re lated sinusoids must be in phase with each other. There must be no relative phase drift between the separate harmonically related sinusoids, and there must be no persistent phase lags or leads. These conditions are required in order to maintain the distinguishability of the waveforms and to eliminate, insofar as possible, discontinuities between the product waveforms in the output product waveform train.

Second, the gating pulses from the encoder must be synchronized with the harmonically related sinusoids in such a way that the product waveform gated out during each time interval starts and ends on the zero axis. This condition will provide for a continuous product waveform train having few discontinuities. In particular, it has been found that gating pulses beginning at a time given by T(n1/2) and ending at a time given by T(n+1/Z) give a set of product waveforms large numbers of which begin and end on the zero axis. (T=the period of the fundamental frequency, 11:0, 1, 2, 3 FIG. 5 shows the proper phase relationships between the component harmonically related sinusoids and the gating pulses.

Proceeding now to the receiving end of the communication system, FIG. 3 shows a detailed block diagram of a product waveform detector suitable for use in the singledigital mode. Synchronization of the product waveform detector to the incoming train of product waveforms is accomplished by means of a composite extractor 60. Such a composite extractor may be of a type weil known in the art. For example, the composite extractor might comprise a full-wave rectifier detecting the deep notches in the envelope of the composite waveform which occur at the beginning or end of all transmission intervals. This is due to the fact that most product waveforms start and end at zero levels. These notches occur with a regular periodicity of 1/T. This repetition rate can be extracted with a flywheel synchronization circuit such as are used in television receivers.

The presence of such notches may be expressed more markedly by the insertion of short synchronization gaps between any two transmission intervals. If amplitude modulation of a carrier signal is contemplated, the carrier may be keyed to zero level during the synchronization gaps, while care may be taken that the negativ peaks of the composite waveform may never reach down the zero carrier level. This system gives clear synchronization pulses after envelope detection which may be easily separated from the product waveforms by simple clipper circuits. This method is the well known synchronization pulse separation method of conventional television broadcasting methods. (See for example: A. V. T. Martin, Technical Television, Prentice Hall, 1962, or S. W. Amos and D. C. Birkinshaw, Television Engineering, Principles and Practice; Iliffe Books Ltd., London, 1962.)

The output from the composite extractor is fed into the frequency synthesizer 50 on line 75 in order to control the phase of the synthesizer output with respect to the incoming train of product Waveforms. The frequency synthesizer 50 produces a set of eight harmonically related sinusoids identical to the sinusoids produced in the frequency synthesizer 10 shown in FIG. 2. Selected ones of the set of harmonically related sinusoids are patched into the multipliers 41-44 so as to produce the same set of product waveforms that are produced by multipliers 3134 shown in FIG. 2. During each time interval the received product waveform is multiplied with each of the product waveforms by means of multipliers 41-44. The outputs from multipliers 41-44 are integrated over the time interval by means of integrators 61-64 connected to the ouput from each multiplier 41-44. The outputs from the integrators 61-64 are then compared in the decision device 70 which serves to determine the largest output.

Where orthogonal product waveforms are employed and low channel noise conditions exist only one integrator will produce an output, and the outputs from the other integrators will be zero. Under these conditions, the job of decision device 70 is simple. However, where the waveforms are only quasi-orthogonal or where significant amounts of channel noise are present, each of the integrators 61-64 will produce an output pulse during each time interval. Only the largest of these pulses will correctly identify a received product waveform. Therefore, a largest-ofm-decider is required in order to determine which of the pulses is the largest and pass only that pulse to the decoder. Largestbf-nz-deciders are well known in the art. For example, U.S. Patent 2,815,448 of the present inventor discloses an auction circuit which performs the functions of the largest-ofm-decider shown in FIG. 3. An additional reference is A Limit for Probability of Error for Largest of Selection, by Frank G. Splitt, IEEE Trans. Commun. Syst., vol. CS-l l, December 1963, pp. 494496.

In some situations it might be found desirable to employ a maximum likelihood decider in place of the largestof-m-decider 70 shown in FIG. 3. Maximum likelihood deciders are discussed in Introduction to Statistical Communication Theory, Dave Middleton, McGraw Hill, New York, N.Y., 1960. Under certain symmetrical operating conditions a maximum likelihood decider becomes identical to a largest-of-m-decider.

It is noted that decision device 70 receives control inputs 76 and 77 from composite extractor 60. Control input 76 serves a timing function. More specifically, control input 76 serves to place the decision instant exactly at the end of each received waveform. Control input 77 serves to adjust the decision threshold of decision device 7%] in order to compensate for variations in the power of the received signal due, for instance, to transmission channel attenuation.

It is further noted that decision device 70 produces control outputs on lines 78 and 79. Output 79 carries signals for updating the timing of frequency synthesizer 50. Such signals may be derived from a means for sensing the exact location of the correlation peaks on an average basis. This peak sensing function may be accomplished by means of a differentiator and a zero crossing indicator. Output 78 serves to discharge or clear the integrators 6l64 after each decision is made by decision device 70.

FIG. 4 shows a product waveform detector adapted for use in the multi-digital mode. The structure of this form of product waveform detector is the same as that shown in FIG. 3 except that the largest-of-m-decider 70 shown in FIG. 3 is replaced by individual threshold devices 7l 74 at the output of each integrator 6164. If, at the end of a transmission interval the output from any of the integrators 61-64 exceeds the threshold level of its associated threshold device 7174, a pulse will be produced at the output 8184 of the threshold device. The output from the composite extractor 60 makes the necessary adjustments in the threshold levels during each transmission interval.

Threshold devices such as they are used in FIG. 6, 71- 74, are well known in the art. Indeed a whole new branch of switching theory is based on such devices. (See for example: H. F. Klock, G. D. Kraft and R. D. Haney: Logic of Controlled Threshold Devices; NASA document N63-16944, Case Institute of Tech, Cleveland, Ohio, December 1962.) The practical circuits use TRL (transistor-resistor-logie) techniques (See: W. T. Wray, Jr.: Worst Ease Design of Variable Threshold TRL Circuits, IRE Trans. Electronic Computers, vol. EC 11, June 1962, pp. 382-390) or they use tunnel diodes. (See for example: G. A. Rogers: A Base Clipping Technique Using Tunnel Diodes, with Automatic Adjustment of Clipping Level, Report RRE TN 704, Royal Radio Establishment, Malvern, England, July 1963, ASTIA document AD 424003).

Operation of system using two-factor product waveforms The principles of operation of my invention may be best understood by reference to the operation of a specific communication system utilizing my invention. For example, referring to FIG. 1, let us assume that the input information is in the form of a train of amplitude modulated pulses. Further, let us assume that the particular encoder 1 serves to quantize each pulse into one of four possible levels. Hence, for each amplitude modulated pulse that appears at the input to the encoder 1, a gating pulse appears on the one of the four outputs of the encoder 1 which corresponds to the particular amplitude level of the input pulse. For example, assume that a gating pulse appears on line 23 of the product waveform generator shown in FIG. 2. Such a gating pulse would cause a product waveform corresponding to waveform C shown in FIG. 6 to appear at the output of multiplier 33 shown in FIG. 2. This waveform would then be introduced into the transmission channel for transmission to the receiving apparatus.

At the receiving apparatus, the received product waveform is applied to the inputs of each of the multipliers 41-44. The other inputs to the multipliers comprise combinations of harmonically related sinusoids selected so as to form product waveforms corresponding to those shown in FIG. 6. Since the received product Waveform corresponds to waveform C shown in FIG. 6, only the output from multiplier 43 will be positive over the entire time interval over which the received waveform is multiplied with each of the locally generated product waveforms. The outputs from the other multipliers 41, 42 and 44 will assume both positive and negative values over the same time interval. As a result, the output from integrator 63 will have a large positiv value at the end of the time interval. On the other hand, the output from integrators 61, 62, and 64 will have zero value in the case of orthogonal waveforms and no channel noise, and either positive or negative values of low absolute value in the case of quasi-orthogonal waveforms and/or channel noise.

In the largest-of-m-decider 70 the outputs from the integrators are compared and the output from integrator 63 is identified as the largest. Correspondingly output 83 of largest-of-m-decider 70 is activated during this time interval. The decoder 7 operates to transform this information into any desired form. For example, it may be desired to convert this information back into a train of amplitude modulated pulses corresponding to the original input train. Alternatively. it might be found desirable to convert this information to pulse position modulated information, frequency modulated information, analog information, etc. Any of these decoding functions can be performed by decoders which are well known in the art.

An alternative communication system might operate in the multi-digital mode utilizing the principle of superposition of orthogonal waveforms. For example, assume that the original input data is in the form of serial binary coded decimal digits. Given this input, the encoder 1 shown in FIG. 1 would perform the function of a serial to parallel converter. During each time interval, a combination of gating pulses would appear on the inputs 21 through 24 to the product waveform generator shown in FIG. 2. For example, suppose that during a particular time interval gating pulses appear on lines 22 and 24, corresponding to the BCD representation of the decimal digit 5. In this case product waveforms corresponding to waveforms B and A would appear at the outputs of multipliers 32 and 34 respectively. These waveforms would be superposed to form a composite waveform at the output 39. This composite waveform would then be introduced into the transmission channel. At the receiving apparatus the received composite waveform would be introduced into the product waveform detector on line 40 shown in FIG. 4. The received composite of the two prod uct waveforms is then introduced to each of the multipliers 41-44. The other inputs to the multipliers 41-44 are combinations of harmonically related sinusoids selected so as to form product Waveforms corresponding to those shown in FIG. 6. During the time interval over which the waveforms are multiplied, the outputs from multipliers 42 and 44 should be predominantly positive, whereas the outputs from multipliers 41 and 43 should be both positive and negative. As a result, the outputs from integrators 62 and 64 should have very large positive values at the end of the time interval. On the other hand, the outputs from integrators 61 and 63 will be of low absolute value and may be either positive or negative in polarity. The large values of the outputs of integrators 62 and 64 should exceed the threshold values of threshold devices 72 and 74 causing them to produce output pulses. On the other hand, the low values of the outputs from integrators 61 and 63 should not exceed the threshold values of threshold devices 71 and 73, hence threshold devices 71 and 73 would produce no output pulses. It is noted that a threshold adjusting device responsive to the magnitude of the composite signal is required in order to adjust the individual threshold levels of the threshold devices 71-74 because, if a large number of product waveforms form the composite waveform, the composite waveform will be of large magnitude. This large magnitude may cause a spuriously high output from one of the integrators 61-64 when no correlation is actually desired. Hence, it is desirable to raise the threshold levels of the individual threshold devices when the magnitude of the composite waveform is large.

The outputs from the individual threshold devices are applied to the decoder 7 shown in FIG. 1. In the particular communication system described, it might be found desirable that the decoder perform the function of a parallel to serial converter so as to convert the received information back into a series of binary coded decimal digits corresponding to the form of the original input data.

In the foregoing description of both the single-digital and multi-digital communication systems, it is noted that a product waveform is transmitted in the positive sense when a binary 1 appears at one of the inputs 21-24 of FIG. 2. When a binary 0 appears, no waveform is transmitted. Another mode of operation involves the transmission of product waveforms in both positive and negative senses. This mode of operation is sometimes called the biorthogonal mode.

One example of the bi-orthogonal mode of operation comprises the transmission of a product waveform in the positive sense when a binary 1 appears at its associated input, and the transmission of the same product waveform, but with its polarity reversed, when a binary 0 appears at the input. If this mode of operation is to be used, certain modifications of the product waveform detectors might be desirable. For example, in the product waveform detector shown in FIG. 4, threshold devices 71-74 might be provided with means for sensing both positive and negative threshold levels. For example, if the output from one of the integrators 61-64 exceeds the positive threshold level, a binary 1 is indicated. If the output from the integrator exceeds the negative threshold level, then a binary 0 is indicated. If neither threshold is exceeded then an 

